(1) Field of the Invention
The present invention relates to a system and a method for low-cost and high-performance absolute position referencing for elevators and other conveyances. More specifically, the present invention relates to a system incorporating both RFID sensing technology and CCD image processing to accurately assess the position of a moveable platform.
(2) Description of Related Art
A Positioning Reference System (PRS) is a component of an elevator control system that provides fast and accurate position measurement of a moveable platform, in particular an elevator car, in a hoistway. Traditional optical elevator position reference systems (PRS) exhibit a number of attributes that render them less than operationally optimal.
One method of employing a PRS comprises using a coded tape defined by two tracks of openings cut along the length of the tape. The coded tape spans the entire longitudinal length of a hoistway. An optical sensor, mounted on the elevator car, consisting of a light transmitter and a receiver reads the binary coded information from the coded tape, and relate the readings to the actual position of the elevator car in the hoistway.
A disadvantage of this system is that longitudinal expansions of the hoistway as well as the coded tape can cause positioning errors. Another disadvantage of this system is the high installation costs involved in the installing the tape in the hoistway.
An alternative method involves the use of reflectors mounted on the wall of a hoistway having two tracks of coded symbols printed on reflectors for elevator car positioning. The symbols are detected and evaluated by a two-channel evaluating subsystem, which has optical transmitters for illuminating the tracks and charge-coupled device (CCD) sensors for detecting the reflected images. Pattern recognition logic for each channel recognizes patterns in the images and generates elevator car position information.
A disadvantage of this system is that it requires the use of two CCD cameras for generating elevator car position: one for reading the floor coarse position information and other one for fine positioning information. If redundant systems have to provide data for the normal terminal stopping device (NTSD) and the emergency terminal stopping device (ETSD), the resultant system configuration would cause the production cost to be quite high. Since the system is intended to provide linear position feedback along the hoistway, the coding of the white and dark patterns on the reflector has to be very fine. Very fine coding patterns make the system susceptible to dirt and dust contaminations that may blur the coding patterns causing inaccurate car position information. Another disadvantage is the great effort required for installing the reflector along the path in the elevator hoistway.
What is therefore needed is a high-accuracy positioning means with low cost for installation, and maintenance.